Single-phase induction motors are widely used, due to their simplicity, strength and high performance. They are generally found in household appliances, such as refrigerators, freezers, air conditioners, hermetic compressors, laundry machines, pumps, fans, and in some industrial applications.
The known induction motors are usually provided with a cage type rotor and a coiled stator having two windings, one for the running coil and the other for the starting coil. During the normal operation of the compressor, the running coil is supplied by an AC voltage and the starting coil is temporarily supplied at the beginning of the starting operation, creating a turning magnetic field in the air gap of the stator, a necessary condition to accelerate the rotor and start the motor.
This turning magnetic field can be obtained by supplying the starting coil with a current that is time-displaced in relation to the current circulating in the main coil, preferably at an angle close to 90 degrees. This time-displacement between the current circulating in both coils is achieved by constructive characteristics of the coils or by installing an external impedance in series with one of the coils, but generally in series with the starting coil. The value of this current circulating through the starting coil during the starting process of the motor is generally high, requiring the use of some type of switch which can interrupt this current after the time required to promote motor acceleration has elapsed.
For the motors in which very high efficiency is required, this starting coil is not completely switched off after the starting period has elapsed and a capacitor, namely a running capacitor, is maintained connected in series with said starting coil, providing enough current to increase the maximum torque of the motor and its efficiency.
For the motors with this construction, using a permanent impedance in series with the starting coil during the normal operation of the motor, there are known some starting devices, such as PTC, electromechanical relay, timers, or combinations in which a PTC is connected in series with a device that interrupts the passage of current after a determined time (RSP), as described in U.S. Pat. Nos. 5,053,908 and 5,051,681 and in the international co-pending patent application WO02/09624A1 of the same applicant.
One of the components widely used for starting the motors of the split phase type, in which a running capacitor is not used, is the electromechanical relay. Its wide use is related to its low manufacturing cost and simple technology. On the other hand, the electromechanical relay presents several limitations, especially the need of dimensioning a specific component for each size of electric motor, the impossibility of using it in high efficiency motors in which a running capacitor is used, generation of electromagnetic and other noises during operation of the motor and wear of its components caused by electric arc and mechanical friction.
An alternative for the electromechanical relay is the PTC (positive coefficient temperature) device. This component is widely used in high efficiency motors whose application is associated with a running capacitor. Since such component is a ceramic tablet having no movable parts, its principle overcomes most limitations of the electromechanical relay. Since its operation is based on heating a ceramic tablet, increasing its electrical resistance and consequently limiting the circulating current, there occurs dissipation of residual power during all its operating time. Another limitation of this component is related to the time interval required to allow consecutive starts to occur.
One of the great advantages is the possibility of using a single component to operate in the start of a family of motors with a determined voltage (115V or 220V), but which becomes a limitation when the analyzed characteristic is the optimization of the energizing time of the auxiliary coil. Its conduction time is directly proportional to the volume of the ceramic tablet and inversely proportional to the circulating current, resulting in a reduced starting time when applied to high power motors, and in an excessively long time when applied to smaller motors. These two facts lead to a deficiency in the start of the larger motors and to higher energy consumption during the starting period of the smaller motors.
On the other hand, the timed starting components eliminate the great disadvantage of the PTC residual consumption, but do not overcome the deficiency in the starting time required for the different motor sizes. Its concept permits dimensioning a circuit which provides the optimized starting time for the different motor sizes. Thus, it is mandatory the existence of several components to comply with a family of electric motors, resulting in products of different models, adjustments in the production lines, and higher inventories. Such devices disregard the operating conditions in the starting instant and are thus dimensioned for the worst condition, increasing the starting time.